The invention relates to a method for determining the deviations in the measured geometric dimensions and/or position of an object from defined desired values of said geometric dimensions and/or position of the object. The objects or elements or articles may be of varying dimensions.
Prior art coordinate measuring machines for automatically measuring workpieces already exist. With these methods, the workpieces are scanned continuously, e.g. by means of a tracer finger. The measured values obtained then undergo further processing (DE 35 23 188 A1). Optical measuring systems are also used to determine the coordinate values of, for example, three-dimensional objects. For non-contact scanning an optical measuring head is used: its movements are controlled and its output signals are processed by a computer as the object is scanned. The measured values are analysed using software. The analysis includes, for example, a comparison of the desired/actual dimensions of the measured object. The results are used for quality control purposes, for example. The above method is also used to measure free-form surfaces and compare them with defined desired surfaces. To determine the deviations between desired and actual values, the measured values are adjusted to the desired geometries. The deviations are determined after said adjustment. The deviations are compared with the defined tolerances of the desired surfaces to determine whether the elements or objects are within the tolerance limits, or the points at which tolerance limits are exceeded. Because of the adjustment to the desired geometry, it is impossible to take functional tolerance zones into account.
In measurement technology, adjustment is a well known method where a desired contour and an actual contour are displaced and/or rotated until both contours are aligned xe2x80x9cas best as possiblexe2x80x9d. Reference is made in this context to a website on the Internet which, although it can be accessed now, relates to a project carried out between 1994 and 1997, i.e. prior to the filing date. By way of further proof we also enclose offprint October 1996 from xe2x80x9cKontrollexe2x80x9d containing an essay on xe2x80x9cKonturscanningxe2x80x9d [Contour Scanning] by W. Grebe. The scanning-best fit method is described on the last page of this offprint in a section entitled xe2x80x9cDatenauswertungxe2x80x9d [Data Evaluation], and in FIG. 7.
With the adjustment technique, the position of a measured actual contour is therefore adjusted by rotation and/or translation in respect of certain optimising parameters so that it more or less coincides with the position of a desired contour as per the pre-defined parameters.
The general aim is to align the desired and actual positions so that the deviations between both contours are minimal. In this sense the optimum position between the desired contour and the actual contour is determined by a process of calculation. This is no academic calculation of measured values, but a genuine reflection of the practical field application which shows the true deviations between desired and actual values, form and dimension, andxe2x80x94where the rotation and displacement parameters are recordedxe2x80x94also the true position between both.
All that is done with the prior art adjustment methods, including for example the xe2x80x9cWerth Best Fitxe2x80x9d method, is to optimise the position of the desired and actual contours.
A computer-integrated measuring system is know from DE 37 25 347 for determining the deviations in the measured geometric dimensions of an object from defined desired values of said geometric dimensions of the object. An adaptation of the desired values of the geometric dimensions under consideration of the default tolerance values of said desired values has to be implemented before the deviations are determined.
The object underlying the present invention is to develop a method for determining the deviations in the measured geometric dimensions and/or position of an object from the defined desired values for said geometric dimensions and/or position of the object, with which, by improving the adjustment between the measured values, it can be determined with greater accuracy whether defined limit values in the dimensions and/or the position of the measured object are exceeded, so that functional tolerance zones can be taken into account. In accordance with the invention, the problem is substantially solved with the method described at the outset in that before determination of the deviations, the measured values of the geometric dimensions of the object in question and/or the position of the object are adjusted to the desired values whilst taking into account the defined tolerances for the desired values of the geometric dimensions and/or the position of the object. The invention is based on the principle that the measured values, which will be referred to below as the actual values, should be adjusted to the desired values of the object in such a way that none of the measured values, of which there should be as few as possible, are outside the measured value tolerance zone, although inside the tolerance zone it is not necessary for the distances with respect to the desired values to be at their minimum. With the method in accordance with the invention, the quality control results obtained by measuring objects and comparing the measured values with defined desired values are significantly improved. After the adjustment process, it is determined and signalled during the evaluation of just a few tolerance non-conformities when the object in question is optimally positioned within the tolerance zone. In accordance with the invention, the minimum deviation in the measured values from the defined desired values is a criterion that, during the adjustment process, is modified with reference to the deviation in at least one part of the measured values with respect to the tolerance values attributed to the desired values in question. When this modification is carried out, other deviations between desired and actual values may occur in comparison with the adjustment, which focuses only on the desired values. After adjustment, the actual values are checked with regard to their position relative to the desired values and the tolerances attributed to the latter.
With this invention, account is taken during the adjustment process of defined tolerances attributed to individual contour sections of the desired contour. The adjustment process, which consists of rotation and displacement by a computer, for example, then supplies a result which is particularly suitable for checking the fit of the actual contour.
One preferred embodiment provides for the measured values to be attributed point-by-point as actual values to the corresponding desired values, for the differences between the actual values and the tolerances attributed to the corresponding desired value points to be determined, and for the adjustment to be carried out in relation to zero or minimum deviation in the actual values from the tolerances. By tolerances are meant the limit values at each desired value, i.e. the difference between the precise desired value point and the attributed limit value of the tolerance, which is pre-defined and must not be exceeded for whatever reason, e.g. as a quality criterion.
In particular, the adjustment of the actual values to the pre-defined tolerance field is earned out according to the Gaussian method. This ensures that a good adjustment is achieved.
Another preferred embodiment for the adjustment provides for a non-linear optimisation between actual values and tolerance values.
The adjustment method in accordance with the invention is preferably used for objects which, in the three-dimensional space, have one or more arbitrarily oriented contours or planes. The objects may have one or several ruled geometries or free planes in space. The planes or surfaces may have different tolerance zones.
It is preferable if, after the adjustment process, the deviations from the tolerances are calculated and indicated numerically and/or displayed graphically. The method in accordance with the invention is especially suited for aligning three-dimensional coordinate measurements.